1. Field of the Invention
This invention relates broadly to vascular prostheses. More particularly, this invention relates to coronary stents.
2. State of the Art
Cardiovascular disease, including atherosclerosis, is one of the leading causes of death in the United States. A number of methods have been developed for treating coronary heart disease, and particularly for treating complications resulting from atherosclerosis and other forms of coronary arterial narrowing.
One of the most important recent developments for the treatment of atherosclerosis and other forms of coronary vascular narrowing is percutaneous transluminal coronary angioplasty, hereinafter referred to as "angioplasty". The angioplasty procedure enlarges the lumen of the affected coronary artery by radial hydraulic expansion, and is accomplished by inflating a balloon within the narrowed lumen of the coronary artery. Radial expansion of the coronary artery by the balloon operates to flatten soft plaque, crack and split hardened deposits, and stretch the wall of the artery to a greater diameter.
Angioplasty is typically performed by first introducing a thin-walled hollow guiding catheter into the body via a relatively large arterial vessel, such as the femoral artery in the groin area. The guiding catheter is advanced through the femoral artery, into the iliac artery, into the ascending aorta, negotiated through the sharp turn of the aortic arch, and descended into the aortic cusp. Once at the aortic cusp, the guiding catheter may be moved to either of the left or right coronary arteries.
After the guiding catheter is advanced to the ostium of the coronary artery to be treated by angioplasty, a flexible guidewire is inserted through the guiding catheter to the location to be treated and extended through the area of constriction. A balloon catheter is then advanced over the guidewire and inflated at the locus of the constriction.
Unfortunately, while the affected artery can be enlarged, in some instances the vessel restenoses chronically, or closed down acutely, negating the positive effect of the angioplasty procedure. In the past, such restenosis has required the angioplasty procedure to be repeated or necessitated open heart surgery. While restenosis does not occur in the majority of cases, it occurs frequently enough such that such complications comprise a significant percentage of the overall failures of the angioplasty procedure, for example twenty-five to forty percent of such failures.
To lessen the risk of restenosis, stents have been used across the vascular lesion to mechanically maintain a previously balloon-expanded blood vessel in a dilated condition. In general, a stent is advanced to the site of the lesion in a constricted diameter state, and then expanded in the lesion to keep the vascular passageway open. Effectively, the stent overcomes the natural tendency of the vessel walls of some patients to close back down, thereby maintaining a more normal flow of blood through the blood vessel than otherwise possible.
Various types of stents have been used, generally falling into two categories: pressure-expandable and self-expanding. Pressure-expandable stents are typically formed of a malleable material, e.g., stainless steel or tantalum, and are deployed on an inflatable balloon or other expanding member such that upon inflation of the balloon or expansion of the expanding member, the stent will be radially enlarged to a desired diameter such that the stent becomes positioned against the surrounding blood vessel wall. Pressure-expandable stents are disclosed in U.S. Pat. Nos. 5,135,336 to Hulstead; 4,733,685 to Palmaz; 4,922,905 to Strecker; 4,950,227 to Savin et al.; 5,041,126 to Gianturco; 5,108,416 to Ryan et al.; 5,217,483 and 5,161,547 to Tower; and 5,443,496 to 5,443,496 to Schwartz et al. Pressure-expandable stents are generally formed from either a tube of the malleable material which is laser-cut to remove portions of the material to create slots or windows in the tube, i.e., a `slotted tube`, or formed of one or more inelastic malleable wires coupled together in a tubular form by one or more links. Either type of stent permits the stent to be diametrically expansibly deformed under pressure.
Pressure expandable stents of the `slotted tube` type do provide a relatively large flattened surface area for contact with the inner surface of the vascular wall, resulting in reduced trauma to the lesion. However, the stents are relatively stiff and hard to maneuver through the tortuous curves of the vessel structure. In addition, the stent implant procedure is complicated, requiring the stent to be sufficiently expanded to prevent the stent from inadvertent release from the location of the lesion, yet limited in its expansion so as to not cause additional injury to the vessel wall. Moreover, pressure-expandable stents do not readily conform to the shape of the vessel at the lesion, and may overstretch the vessel wall at one location while insufficiently supporting the wall at another location. Furthermore, `slotted tube` stents are relatively costly and difficult to manufacture.
Self-expanding stents are generally formed from a spring metal or other resilient material and deployable through a guiding catheter on a delivery catheter covered with a lubricous sleeve. When the sleeve is withdrawn over the self-expanding stent, the stent automatically expands so as to exert pressure against the surrounding blood vessel wall. Self-expanding stents are disclosed in U.S. Pat. Nos. 4,580,568 to Gianturco; 4,830,003 to Wolff et al.; 5,549,635 to Solar; 5,562,697 to Christiansen; and 5,292,331 and 5,674,278 to Boneau. Such stents are typically formed from a single small diameter wire having a multiplicity of back and forth bends in a zig-zag or sinusoidal path to form an elongate self-expanding structure, or a plurality of self-expanding segments coupled by links, each of the segments defined by a wire having a zig-zag or sinusoidal path, or a plurality of plaited wires.
Self-expanding stents tend to have great flexibility, primarily due to the materials used and the structures formed, and can be more easily maneuvered through the curves of blood vessels to the lesion site. However, self-expanding wire stents have been less successful in use, in some cases causing injury at the locus of implantation. This may be because the construction of self-expanding wire stents tends to require the use of very narrow wires, and such construction translates to a smaller surface area of the stent available for contact against the blood vessel wall. It is believed that, as a result of the small surface area, the pressure of the wire against the vessel causes injury to the blood vessel wall. In addition, self-expanding stents do not necessarily have the structural rigidity to provide the requisite support of a damaged coronary blood vessel.